Summary Diel changes in bacterial and cyanobacterial numbers, as well as heterotrophic bacterial production, were examined in two shallow alkaline pools, harbouring dense populations of cyanobacteria (up to 1100 × 10 9 cells l −1 ) and bacteria (up to 500 × 10 9 cells l −1 ). Together with the recorded bacterial production rates (925 µgC l −1 h −1 ), these values are the highest reported for natural aquatic ecosystems. The inv...